1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, a process cartridge and an image forming apparatus used in electrophotographic image formation.
2. Description of the Related Art
As enabling high-speed and high-quality printing, electrophotography is utilized in an image forming apparatus such as duplicator, laser printer, LED printer. An electrophotographic photoreceptor having a photosensitive layer of a photoconductive material formed on a conductive supporting member is known for use in such an image forming apparatus.
For repeatedly and stably forming images of good quality in such an image forming apparatus, it is necessary to control the image-forming condition including charge current, exposure level and development bias in image formation within a suitable range. The image-forming condition is set in accordance with the characteristics of the electrophotographic photoreceptor to be mounted on the image forming apparatus.
Even though produced in the same manner, photoreceptors may vary in their quality in different lots, and owing to the fluctuation of the photoreceptor quality, images formed may also vary in their quality. For example, even when an image-forming condition suitable to a photoreceptor is set, there may be a possibility that the condition may be unsuitable when the photoreceptor is exchanged for a photoreceptor in a different lot, and therefore a problem may occur in that high-quality images could not always be formed stably.
On the other hand, for example, when the design of the photoreceptor used in a processing line is changed for some reason and when the photoreceptor used therein is exchanged for the newly-designed one, then there may occur a problem in that images of good quality could not be formed since the sensitivity and the charging condition of the exchanged photoreceptor differ from those of the formerly-used one and therefore the image-forming condition for the formerly-used photoreceptor could not directly apply to the exchanged one.
When the sensitivity of the photoreceptor used in a processing line has changed, then the level of exposure to be applied to the photoreceptor must be controlled. When the charge potential of the photoreceptor has changed, then the level of charge (amount of charge current) to be applied to the photoreceptor must be controlled. Further, when the background potential, potential after exposure of the photoreceptor has changed, then the development condition must be controlled. Controlling the image-forming condition thereon within an optimum range enables the image forming apparatus to form images of the best quality.
To solve the problems in that situation, an extremely complicated and expensive reconstruction of the image-forming system is indispensable, including, for example, fitting a unit for measuring the surface potential gauge of a photoreceptor to an image forming apparatus or fitting an unit for measuring the toner concentration thereto.
A cylindrical (drum-type) photoreceptor is widely known for electrophotographic image formation, but the working accuracy of the photoreceptor drum is not always satisfactory, and there may occur eccentricity from the rotary fixed axis thereof. In a color image forming apparatus in which plural photoreceptor drum units are disposed in tandem, each drum attains different color image formation and the resulting images are combined to form a color image, the eccentricity from the rotary fixed axis of the photoreceptor drum unit causes color unevenness in combining the individual color images.
To reduce the color unevenness as much as possible, a registration technique has heretofore been investigated, which includes shifting the direction of eccentricity of each photoreceptor drum unit to the same position on the image plane thereof. Examples of such conventional technique are disclosed in JP-A-10-339976, JP-A-11-030893, and JP-A-2003-337459 (U.S. Pat. No. 6,789,795).
However, the registration technique requires unifying the direction of eccentricity by hand in disposing the photoreceptor drums. Therefore, for example, when a photoreceptor is exchanged for a different one, then it may cause a problem in that an image defect such as color unevenness may occur so far as the registration is not again effected by hand. In addition, there may be another problem in that repeated image formation may cause phase shifting of the individual photoreceptor drums and therefore images of good quality could not be stably formed.